Mock objects in JUnit - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a mock object using Mockito.

JUnit

MyService service = Mockito.[1](MyService.class);

Drag options to blanks, or click blank then click option'

Anew

Bspy

Ccreate

Dmock

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'spy' instead of 'mock' creates a spy, not a pure mock.

Using 'new' is not a Mockito method.

Mockito.mock() creates a mock object of the given class.

2fill in blank

medium

Complete the code to stub the method 'getData' to return 'Hello' when called.

JUnit

Mockito.when(service.[1]()).thenReturn("Hello");

Drag options to blanks, or click blank then click option'

AfetchData

BgetData

Cretrieve

Dcall

Attempts:

3 left

💡 Hint

Common Mistakes

Using a method name that does not exist in the service interface.

Forgetting to use Mockito.when() before thenReturn().

The method 'getData' is stubbed to return 'Hello' when called.

3fill in blank

hard

Fix the error in verifying that 'process' method was called once on the mock.

JUnit

Mockito.verify(service, Mockito.[1]()).process();

Drag options to blanks, or click blank then click option'

AcalledOnce()

Bonce()

Ctimes(1)

DverifyOnce()

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'once()' instead of 'times(1)'.

Using 'calledOnce()' or 'verifyOnce()' which are invalid.

Mockito.times(1) verifies the method was called exactly once.

4fill in blank

hard

Fill both blanks to stub 'calculate' method to return 10 when called with argument 5.

JUnit

Mockito.when(service.[1]([2])).thenReturn(10);

Drag options to blanks, or click blank then click option'

Acalculate

B5

C10

Dcompute

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong method name like 'compute'.

Using return value as argument.

The method 'calculate' is stubbed to return 10 when called with argument 5.

5fill in blank

hard

Fill all three blanks to verify 'update' method was called twice with argument 'data'.

JUnit

Mockito.verify(service, Mockito.[1]([2])).[3]("data");

Drag options to blanks, or click blank then click option'

Atimes

B2

Cupdate

Donce

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'once' instead of 'times' with 2.

Mixing method names or call counts.

Mockito.verify with times(2) checks 'update' was called twice with argument 'data'.

Practice

1. What is the main purpose of using mock objects in JUnit testing?

easy

A. To speed up the compilation of test classes

B. To simulate real objects and control their behavior during tests

C. To replace the JUnit framework with a simpler tool

D. To automatically generate test data without coding

Solution

1. Step 1: Understand the role of mock objects

   Mock objects imitate real objects to isolate the code being tested and control responses.

2. Step 2: Identify the main benefit in testing

   This helps test code without relying on actual implementations or external systems.

3. Final Answer:

   To simulate real objects and control their behavior during tests -> Option B

4. Quick Check:

   Mock objects = simulate and control behavior [OK]

Hint: Mocks simulate objects to isolate tests quickly [OK]

Common Mistakes:

• Thinking mocks speed up compilation
• Confusing mocks with test data generators
• Believing mocks replace the JUnit framework

2. Which of the following is the correct way to create a mock object using Mockito in JUnit?

easy

A. MyClass mock = mock(MyClass.class);

B. MyClass mock = new MyClass().mock();

C. Mock<MyClass> mock = new Mock<>();

D. MyClass mock = createMock(MyClass);

Solution

1. Step 1: Recall Mockito syntax for creating mocks

   Mockito uses mock(Class.class) to create mock objects.

2. Step 2: Match the syntax to options

   MyClass mock = mock(MyClass.class); matches the correct syntax: MyClass mock = mock(MyClass.class);

3. Final Answer:

   MyClass mock = mock(MyClass.class); -> Option A

4. Quick Check:

   Mockito mock creation = mock(Class.class) [OK]

Hint: Use mock(Class.class) to create mocks in Mockito [OK]

Common Mistakes:

• Using new Mock<>() which is not valid
• Calling mock() on an instance
• Using createMock which is not Mockito syntax

3. Given the following code snippet, what will result be after execution?

List<String> mockList = mock(List.class);
when(mockList.size()).thenReturn(5);
int result = mockList.size();

medium

A. 5

B. 0

C. Throws NullPointerException

D. Returns the actual size of a real list

Solution

1. Step 1: Understand the mock setup

   The mock list's size() method is stubbed to return 5 when called.

2. Step 2: Evaluate the method call

   Calling mockList.size() returns 5 as per the stub, not the real list size.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   Stubbed method returns set value 5 [OK]

Hint: Stubbed methods return specified values, not defaults [OK]

Common Mistakes:

• Expecting default 0 for int return
• Thinking mock calls throw exceptions by default
• Confusing mock with real object behavior

4. Identify the error in the following JUnit test code using Mockito:

MyService service = mock(MyService.class);
when(service.getData()).thenReturn("data");
verify(service).getData();

medium

A. No error, code is correct

B. Incorrect use of mock() method

C. Cannot verify void methods

D. Missing call to service.getData() before verify

Solution

1. Step 1: Understand verify usage in Mockito

   Verification checks if a method was called on the mock during the test.

2. Step 2: Check if method was called before verify

   The code never calls service.getData() before verify, so verification fails.

3. Final Answer:

   Missing call to service.getData() before verify -> Option D

4. Quick Check:

   Verify requires method call before checking [OK]

Hint: Always call method before verify to avoid errors [OK]

Common Mistakes:

• Verifying without calling the method
• Confusing mock creation syntax
• Thinking verify only works on void methods

5. You want to test a method that calls a dependency twice with different inputs and expects different outputs. How do you configure a mock to return "first" on the first call and "second" on the second call to getValue()?

hard

A. when(mock.getValue()).thenReturn("first"); when(mock.getValue()).thenReturn("second");

B. when(mock.getValue()).thenReturn(new String[]{"first", "second"});

C. when(mock.getValue()).thenReturn("first").thenReturn("second");

D. when(mock.getValue()).thenReturn("second").thenReturn("first");

Solution

1. Step 1: Recall Mockito chaining for multiple returns

   Mockito allows chaining thenReturn() calls to specify sequential return values.

2. Step 2: Match chaining syntax to options

   when(mock.getValue()).thenReturn("first").thenReturn("second"); matches the correct chaining syntax for sequential calls.

3. Final Answer:

   when(mock.getValue()).thenReturn("first").thenReturn("second"); -> Option C

4. Quick Check:

   Chain thenReturn() for multiple call returns [OK]

Hint: Chain thenReturn() calls for sequential mock returns [OK]

Common Mistakes:

• Passing multiple values in one thenReturn() call
• Overwriting stubs instead of chaining
• Reversing the order of return values